New Government policy will limit AstraZeneca rollout to over-30s only, with everyone under the threshold offered an alternative.
Authorities have advised GP-led sites to cancel incoming appointments for this same group.
Professor Wei Shen Lim, The JCVI’s COVID-19 Chair, said his organisation wanted a safety-first approach.
READ MORE: What does it feel like when you have a blood clot?
He said in a statement: “Safety remains our number one priority.
“Based on the available data and evidence, JCVI has advised that it is preferable for adults aged under 30 with no underlying conditions to be offered an alternative to the AstraZeneca vaccine where available.
“This weighs up the risks of being seriously ill or dying from COVID-19 against the extremely small risk of a serious adverse event.”
He added the Government risks low public compliance if it walks back its advice.
Dr Easmon said: “I think the AZ vaccine has lost this race in the UK in that AZ stopped their own trial in the young, and the MHRA and JCVI have advised under 30s be given a different vaccine.
“You cannot ‘walk back’ advice like that and expect public confidence and compliance.”
Just 79 people in the more than 20 million recipients experienced clotting issues.
Of those, 19 people have died, while others spent time in the hospital.
The Medicines and Healthcare products Regulatory Agency (MHRA) estimates an incidence rate of four in one million, close to the non-vaccinated average.